Methods and apparatus for performing spine surgery

ABSTRACT

Systems and methods are described for correcting sagittal imbalance in a spine including instruments for performing the controlled release of the anterior longitudinal ligament through a lateral access corridor and hyper-lordotic lateral implants.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 15/702,685 filed on Sep. 12, 2017, which is a continuation of U.S. patent application Ser. No. 14/918,137 filed Dec. 19, 2014 (now U.S. Pat. No. 9,757,246), which is a continuation of U.S. patent application Ser. No. 14/578,215 filed Dec. 19, 2014 (now U.S. Pat. No. 9,192,482), which is a continuation of U.S. patent application Ser. No. 13/653,335 filed Oct. 16, 2012 (now U.S. Pat. No. 8,920,500), which is a continuation of U.S. patent application Ser. No. 12/799,021 filed Apr. 16, 2010 (now U.S. Pat. No. 8,287,597), which claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 61/212,921 filed Apr. 16, 2009, and U.S. Provisional Patent Application Ser. No. 61/319,823 filed Mar. 31, 2010, the entire contents of which is hereby expressly incorporated by reference into this disclosure as if set forth in its entirety herein.

FIELD

The present invention relates to implants and methods for adjusting sagittal imbalance of a spine.

BACKGROUND

A human spine has three main regions—the cervical, thoracic, and lumbar regions. In a normal spine, the cervical and lumbar regions have a lordotic (backward) curvature, while the thoracic region has a kyphotic (forward) curvature. Such a disposition of the curvatures gives a normal spine an S-shape. Sagittal imbalance is a condition in which the normal alignment of the spine is disrupted in the sagittal plane causing a deformation of the spinal curvature. One example of such a deformity is “flat-back” syndrome, wherein the lumbar region of the spine is generally linear rather than curved. A more extreme example has the lumbar region of the spine exhibiting a kyphotic curvature such that the spine has an overall C-shape, rather than an S-shape. Sagittal imbalance can be a problem in that it is biomechanically disadvantageous and generally results in discomfort, pain, and an awkward appearance in that the patient tends to be bent forward excessively.

Various treatments for sagittal imbalance are known in the art. These treatments generally involve removing at least some bone from a vertebra (osteotomy) and sometimes removal of the entire vertebra (vertebrectomy), in order to reduce the posterior height of the spine in the affected region and recreate lordotic curve. Such procedures are traditionally performed via an open, posterior approach involving a large incision (often to expose multiple spinal levels at the same time) and require stripping of the muscle tissue away from the bone. These procedures can have the disadvantages of a large amount of blood loss, high risk, and a long and painful recovery for the patient. Furthermore, depending upon the patient, multiple procedures, involving both anterior and posterior approaches to the spine, may be required.

The present invention is directed at overcoming, or at least improving upon, the disadvantages of the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements and wherein:

FIG. 1 is a lateral view representing a portion of a sagitally imbalanced lumbar spine lacking the normal lordotic curvature;

FIG. 2 is a side view representing the lumbar spine of FIG. 1 after restoration of the lordotic curvature using a hyper-lordotic fusion implant, according to one example embodiment;

FIG. 3 is a top down view depicting the creation of a lateral access corridor formed with surgical access system through the side of the patient to the target disc space, according to one example embodiment;

FIG. 4 is a perspective view depicting a lateral access corridor formed with a retractor assembly through the side of the patient to the target disc space, according to one example embodiment;

FIG. 5 is a front perspective view of an Anterior Longitudinal Ligament (ALL) resector for safely releasing the ALL through a lateral access corridor, according to one example embodiment;

FIG. 6 is an enlarged from perspective view of the distal portion of the ALL resector of FIG. 5;

FIG. 7 is an enlarged side view of the distal portion of the ALL resector of FIG. 5;

FIG. 8 is an exploded front perspective view of the ALL resector of FIG. 5;

FIG. 9 is a front perspective view of an ALL resector for safely releasing the ALL through a lateral access corridor, according to another example embodiment;

FIG. 10 is a side view of the ALL resector of FIG. 9;

FIG. 11 is an enlarged side view of the distal end of the ALL resector of FIG. 9;

FIG. 12 is an enlarged side view of the distal end of the ALL resector of FIG. 9 having curved finger portions;

FIGS. 13-15 are anterior-front perspective, posterior-rear perspective, and front views of a hyper-lordotic lateral implant according to one example embodiment;

FIGS. 16-17 are posterior-rear perspective and rear views of a hyper-lordotic lateral implant according to another example embodiment; and

FIGS. 18-19 are posterior-rear perspective and rear views of a hyper-lordotic lateral implant according to still another example embodiment.

DESCRIPTION OF A PREFERRED EMBODIMENT

Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The methods and devices described herein include a variety of inventive features and components that warrant patent protection, both individually and in combination.

With reference to FIGS. 1-2, devices and methods of the present invention are utilized to correct sagittal imbalance, including lumbar kyphosis, by increasing the anterior height of the affected spinal area (as opposed to reducing the posterior height). FIG. 1 illustrates a portion of the lumbar spine lacking the standard lordotic curvature. To correct the sagittal imbalance, illustrated in FIG. 2, a hyper-lordotic implant 10 is positioned into the disc space at the appropriate spinal level (e.g. between V1 and V2). An anterior sidewall 12 of hyper-lordotic implant 10 has a height significantly larger than an opposing posterior sidewall 14 such that when the implant is positioned within the disc space the anterior aspects of V1 and V2 are forced apart while the posterior aspects are not (or at least not to the same degree), thus imparting a lordotic curvature into the spine. To allow the anterior aspects of V1 and V2 to separate and receive the hyper-lordotic implant 10, the Anterior Longitudinal Ligament (ALL) that runs along the anterior aspect of the spine may be released or cut (15). According to a preferred method, the implant 10 is implanted through a lateral access corridor formed through the side of the patient. Accessing the targeted spinal site through the lateral access corridor avoids a number of disadvantages associated with posterior access (e.g. cutting through back musculature and possible need to reduce or cut away part of the posterior bony structures like lamina, facets, and spinous process) and anterior access (e.g. use of an access surgeon to move various organs and blood vessels out of the way in order to reach the target site). Accordingly, by accessing the target site via a lateral access approach and correcting the sagittal imbalance without reducing the posterior height (i.e. no bone removal) the high blood loss and painful recovery associated previous methods may be avoided (or at least mitigated).

According to one example, the lateral access approach to the targeted spinal space may be performed according to the instruments and methods described in commonly owned U.S. Pat. No. 7,207,949 entitled “Surgical Access System and Related Methods,” and/or U.S. patent application Ser. No. 10/967,668 entitled “Surgical Access System and Related Methods,” the entire contents of which are each incorporated herein by reference as if set forth herein in their entireties. With reference to FIGS. 3-4, a discussion of the lateral access instruments and methods is provided in brief detail. With the patient 20 positioned on their side, a surgical access system 20 is advanced through an incision 20, into the retroperitoneal space 22, and then through the psoas muscle 24 until the targeted spinal site (e.g. the disc space between V1 and V2) is reached. The access system 26 may include at least one tissue dilator, and preferably includes a sequential dilation system 28 with an initial dilator 30 and one or more additional dilators 32 of increasing diameter, and a tissue retractor assembly 34. As will be appreciated, the initial dilator 30 is preferably advanced to the target site first, and then each of the additional dilators 32 of increasing diameter are advanced in turn over the previous dilator. A k-wire (not shown) may be advanced to the target site and docked in place (for example, by inserting the k-wire into the vertebral disc) prior to, in concurrence with, or after advancing the initial dilator 30 to the target site. With the sequential dilation system 28 positioned adjacent the target site (and optionally docked in place via the k-wire), the retractor assembly 34 is then advanced to the target site over the sequential dilation system 28. According to the embodiment shown, the retractor assembly 34 includes retractor blades 36, 38, 40 and a body 42. With the sequential dilation system 28 removed, the retractor blades 36, 38, and 40 are separated (FIG. 5), providing the lateral access corridor through which instruments may be advanced to prepare the disc space and insert the implant 10. According to one example, the posterior blade 36 may be fixed in position relative to the spine prior to opening the retractor blades. This may be accomplished, for example by attaching a shim to the blade 36 (e.g. via track 44 including dove tail grooves formed on the interior of blade 36) and inserting the distal end of the shim into the disc space. In this manner, the posterior blade 36 will not move posteriorly (towards nerve tissue located in the posterior portion of the psoas muscle). Instead, the blades 38 and 40 will move away from the posterior blade 36 to expand the access corridor. Additionally, nerve monitoring (including determining nerve proximity and optionally directionality) is preferably performed as each component of the access system 26 is advanced through the psoas muscle, protecting the delicate nerve tissue running through the psoas, as described in the '949 patent and '668 application. Monitoring the proximity of nerves also allows the posterior blade 36 of the retractor assembly 34 to be positioned very posterior (all the way back to the exiting nerve roots), thus exposing a greater portion of the disc space than would otherwise be safely achievable. This inturn permits full removal of the disc and implantation of a wider implant and the wider implant makes utilization of the hyper-lordotic implant with a large lordotic angle (e.g. between 20-40 degrees) more practical.

With the lateral access corridor formed (as pictured in FIG. 5) the target site may be prepped for insertion of the implant 10. Preparation of the disc space may include performing an annulotomy, removal of disc material, and abrasion of the endplates and instruments such as annulotomy knives, pituitaries, curettes, disc cutters, endplate scrapers may be used. Additionally, as discussed above, it may generally be necessary to release the ALL in order to create enough flexibility between the adjacent vertebrae (e.g. V1 and V2) to receive the hyper-lordotic implant 10. Unlike an anterior approach (where the great vessels and other tissue lying anterior to the disc space is retracted during the approach), when the target disc is approached laterally, the great vessels remain adjacent to the ALL along the anterior face of the spine. Thus, while cutting the ALL is generally simple and necessary during an anterior approach surgery, cutting the ALL during a lateral approach surgery has typically been unnecessary and can be difficult because of the need to avoid damaging the great vessels. Accordingly, FIGS. 5-14 set forth various example embodiments of ALL resecting instruments for safely releasing the ALL from a lateral approach.

FIGS. 5-8 illustrate one example ALL resector 50. The ALL resector 50 includes a tissue retractor 52 and a sliding blade 68 and functions to both sever the ALL and protect surrounding tissue (e.g. great vessels, nerves etc . . . ) from unwanted damage while the ALL is being severed. The tissue retractor 52 includes a handle 54, hollow shaft 56, and head 58. The head 58 is curved, preferably such that the inside surface 60 complements the curvature of the anterior aspects of the spinal target site. The head 58 may thus be positioned through the lateral access corridor to the spine and such that the curved interior surface 60 nestles around the anterior curved anterior aspect of the spine. The outside surface 62 will form a barrier, protecting tissue along the anterior spine from inadvertent contact with the sliding blade when the ALL is cut. Furthermore, the tissue retractor 52 can be further manipulated move tissue and further expose the anterior aspect of the target site. The hollow shaft 56 includes a central lumen 64 with an opening adjacent the head 58 and another opening at the opposing end such that the sliding blade 54 may travel through the shaft 56.

The sliding blade 68 includes a blade 70 that is secured to the distal end of an extender 72 by way of an attachment feature 74. The attachment feature 74 as shown is similar to known attachment features used for attaching a blade at the end of a scalpel. It will be appreciated that any number of mechanisms may be used to attach blade 70 to extender 72. Blade 70 may be disposable and extender 72 may be reusable. Alternatively, both blade 70 and extender 72 may be reusable or both may be disposable. The blade 70 includes a cutting edge 76 that, when advanced beyond the lumen 64 of shaft 56, cuts through tissue or material situated adjacent the cutting edge 76.

The proximal end of the extender 72 includes a grip 78 that a user may use to manipulate the position of the sliding blade 40 relative to the shaft 56 and head 58. At least one stop feature 80 extends from the outer surface of the extender 72 which engages with a track 66 that extends along a portion of the elongated shaft 56. The track 66 limits the longitudinal travel of the sliding blade 68 relative to the shaft 56 so that the sliding blade 68 remains slidably mated to the tissue retractor 52 without becoming unassembled and such that the blade 70 cannot extend beyond the protective head 58. Additionally, the stop feature 80 restricts rotation of the sliding blade 68 relative to the tissue retractor 52.

FIGS. 9-8 illustrate another example embodiment of an ALL resector 82. The ALL resector 82 includes a handle 84 (for example, a T-handle) located at the proximal end of the elongated shaft 86 and a distal head 88 for resecting the ALL. The distal head 88 includes distally extending first and second fingers 90, 92, which form an opening 94 therebetween. First and second tapered surfaces 96, 98 which extend a distance from the elongated shaft 84 along the fingers 90, 92 enable the distal head 88 to insert gently between tissue. As best shown in FIG. 11, the first finger 90 may be shorter in length than the second finger 92. However, the first and second finger 90, 92 may be provided in any number of length configurations without departing from the scope of the present invention. By way of example, it has been contemplated that the first finger 14 may be completely removed. Alternatively, as pictured in FIG. 12, the fingers 90 and 92 may be curved and have a more substantial width than shown in FIGS. 9-11. The curvature of the first and second fingers allows the distal head 88 to follow closely along the anterior side of the spine and/or along a curved spatula (not shown) positioned adjacent the anterior side of the vertebral body. Though not shown, a user may optionally insert a spatula along the anterior portion of the ALL prior to inserting the ALL retractor 82. The spatula may serve as additional protection between the delicate tissue anterior to the ALL and the cutting blade 97 of the ALL resector 82. With a spatula in place the user may insert the distal head 88 such that it approaches the lateral side of the ALL and is guided along the inside edge of the spatula. By way of example, the spatula may be straight or curved to match the selected fingers of the distal head 88.

A cutting blade 97 is exposed between the first and second fingers 90, 92 in the opening 17. A slot 99 formed along a side of the distal head 88 allows a cutting blade 97 to be inserted and removed from the distal head 88. Thus, the cutting blade 97 may be disposable and the remainder of the ALL resector 82 may be reusable. Alternatively, both cutting blade 97 and remainder of the ALL resector 82 may be reusable or both may be disposable. In use, the ALL resector 82 is preferably positioned such that the second finger 92 is aligned along the anterior side of the ALL and the first finger 90 is aligned along the posterior side of the ALL, thus, at least partially bounding the ALL on either side. The ALL resector 82 is advanced forward so that the cutting blade 97 cuts through the ALL from one lateral edge to the other. As discussed above, the second finger 92 is preferably aligned along the anterior side of the ALL as the distal head 88 is advanced, thereby shielding the tissue lying anterior to the finger 92 (e.g. great vessels, etc . . . ) from the cutting blade 97. Furthermore, as the user advances the ALL resector 82, the fingers 90, 92 may also act as a stabilizing guide.

Turning now to FIGS. 13-19, various embodiments of the hyper-lordotic implant 10 are described. FIGS. 13-15, for example, illustrate an example implant 110. Implant 110 may preferably be comprised of any suitable non-bone composition having suitable radiolucent characteristics, including but not limited to polymer compositions (e.g. poly-ether-ether-ketone (PEEK) and/or poly-ether-ketone-ketone (PEKK)) or any combination of PEEK and PEKK. Other materials such as for example, metal, ceramics, and bone may also be utilized for the implant 110. Implant 110 has a top surface 116 and bottom surface 118 for contacting V1 and V2, anterior sidewall 112, posterior sidewall 114, and front or leading side 120, and rear or trailing side 122. As discussed, the anterior sidewall 112 has a height greater than the posterior sidewall 114 such that the top surface 116 and bottom surface 118 converge towards each other in the posterior direction. By way of example, the top and bottom surfaces may converge at an angle between 20 and 40 degrees. It is contemplated that variations of the implant 110 may be simultaneously provided such that the user may select from different available ranges. For example, variations may be provided with 20 degree, 30 degree, and 40 degree angles. The top and bottom surfaces may be planar or provided as convex to better match the natural contours of the vertebral end plates. The length of the implant 110 is such that it may span from one lateral aspect of the disc space to the other, engaging the apophyseal ring on each side. By way of example, the implant 110 may be provided with a length between 40 mm and 60.

Having been deposited in the disc space, the implant 110 facilitates spinal fusion over time by maintaining the restored curvature as natural bone growth occurs through and/or past the implant 10, resulting in the formation of a boney bridge extending between the adjacent vertebral bodies.

The implant 110 may be provided with any number of additional features for promoting fusion, such as apertures 127 extending between the top and bottom surfaces 116-117 which allow a boney bridge to form through the implant 110. Various osteoinductive materials may be deposited within the apertures 127 and/or adjacent to the implant 110 to further facilitate fusion. Such osteoinductive materials may be introduced before, during, or after the insertion of the exemplary spinal fusion implant 110, and may include (but are not necessarily limited to) autologous bone harvested from the patient receiving the spinal fusion implant, bone allograft, bone xenograft, any number of non-bone implants (e.g. ceramic, metallic, polymer), bone morphogenic protein, and bio-resorbable compositions, including but not limited to any of a variety of poly (D,L-lactide-co-glycolide) based polymers. Visualization apertures 128 situated along the sidewalls, may aid in visualization at the time of implantation and at subsequent clinical evaluations. More specifically, based on the generally radiolucent nature of the preferred embodiment of implant 110, the visualization apertures 128 provide the ability to visualize the interior of the implant 110 during X-ray and/or other suitable imaging techniques. Further, the visualization apertures 128 will provide an avenue for cellular migration to the exterior of the implant 110. Thus the implant 110 will serve as additional scaffolding for bone fusion on the exterior of the implant 110.

The implant 110 also preferably includes anti-migration features designed to increase the friction between the implant 110 and the adjacent contacting surfaces of the vertebral bodies V1 and V2 so as to prohibit migration of the implant 110 after implantation. Such anti-migration features may include ridges provided along the top surface 116 and/or bottom surface 118. Additional anti-migration features may also include spike elements 124 disposed along the top and bottom surfaces.

Tapered surfaces 126 may be provide along the leading end 120 to help facilitate insertion of the implant 110. Additional instrumentation may also be used to help deploy the implant 110 into the disc space. By way of example, the implant installation device shown and described in detail in the commonly owned and copending U.S. patent application Ser. No. 12/378,685, entitled “Implant Installation Assembly and Related Methods,” filed on Feb. 17, 2009, the entire contents of which is incorporated by reference herein, may be used to help distract the disc space and deposit the implant therein.

FIGS. 16-17 illustrate an implant 210 according to another example embodiment. The implant 210 share many similar features with the implant 110 such that like feature are numbered alike and repeat discussion in not necessary. The implant 210 differs from the implant 110 in that a trailing side 212 is configured for fixed engagement to one of the adjacent vertebral bodies (i.e. V1 or V2) to supplement the anti migration features and ensure the hyper-lordotic implant is not projected out of the disc space. Specifically, the implant 210 includes a tab 213 extending vertically above the top surface 116 and/or below the bottom surface 118. Tabs 213 facilitate fixation of the implant 210 to the adjacent vertebrae. In the example shown in FIGS. 16-17, the tab 213 extends past both the bottom and top surfaces 118 and 116, respectively. It will be appreciated, however, that the implant 210 may include, without departing from the scope of the present invention and by way of example only, a tab 213 that extend solely in one direction. Additionally, multiple tabs 210 of various sizes and/or shapes may be used throughout a single implant 210 to facilitate implantation or fixation. In the illustrated example, the tab has a single fixation aperture 214 for enabling the engagement of a fixation anchor (not shown) within the vertebral bone to secure the placement of the implant. In use, when the implant 210 is positioned within the disc space, the tab 213 engages the exterior of the upper and lower vertebra and the fixation anchor may be driven into the side of either the upper or lower vertebra. One will appreciate that various locking mechanism may be utilized and positioned over or within the fixation aperture 214 to prevent the fixation anchor from unwanted disengagement with the implant 210. For example, a suitable locking mechanism may be in the form of a canted coil disposed within the fixation aperture 214 or a screw that may be engaged to the trailing end 212 and cover all or a portion of the fixation aperture 214 after the fixation anchor positioned.

FIGS. 18-19 illustrate an implant 310 according to another example embodiment. The implant 310 shares many similar features with the implants 110 and 110 such that like feature are numbered alike and repeat discussion in not necessary. The implant 310 differs from the implant 110 in that a trailing side 312 is configured for fixed engagement to one of the adjacent vertebral bodies (i.e. V1 or V2) to supplement the anti migration features and ensure the hyper-lordotic implant is not projected out of the disc space. Specifically, the implant 310 includes a tab 313 extending vertically above the top surface 116 and/or below the bottom surface 118. Tab 313 facilitates fixation of the implant 310 to the adjacent vertebrae. Tab 313 extends past both the bottom and top surfaces 118 and 116, respectively. It will be appreciated, however, multiple tabs 310 of various sizes and/or shapes may be used throughout a single implant 310 to facilitate implantation or fixation. The implant 310 differs from the implant 210 in that the tab 213 has includes a pair of fixation apertures 214 for enabling the engagement of a pair fixation anchors (not shown) within the vertebral bone to secure the placement of the implant. In use, when the implant 310 is positioned within the disc space, the tab 313 engages the exterior of the upper and lower vertebra and a fixation anchor is driven into the side of each of the upper or lower vertebra. One will appreciate that various locking mechanism may be utilized and positioned over or within the fixation apertures 314 to prevent the fixation anchors from unwanted disengagement with the implant 310. One will also appreciate that single locking mechanism may be used to cover both fixation apertures 314, or, a separate locking mechanism may be used to cover each fixation aperture 314. Again, by way of example, a suitable locking mechanism may be in the form of a canted coil disposed within each fixation aperture 314 or a screw that may be engaged to the trailing end 312 and cover all or a portion of each fixation aperture 314 after the fixation anchors are positioned.

While this invention has been described in terms of a best mode for achieving this invention's objectives, it will be appreciated by those skilled in the art that variations may be accomplished in view of these teachings without deviating from the spirit or scope of the invention. For example, particularly at L5-S1 where the pelvic bone makes a lateral access approach difficult, an antero-lateral approach similar to the approach utilized during appendectomies may be utilized. 

What is claimed is:
 1. A method for correcting sagittal imbalance of a spine, the method comprising: creating an operative corridor along a lateral, trans-psoas path to a target site of the spine; preparing an intervertebral space at the target site of the spine, the intervertebral space being at least partially defined by an anterior aspect, a posterior aspect, and opposing first and second lateral aspects; advancing a tissue retractor through the operative corridor along the lateral, trans-psoas path to a protective position in which a protective head of the tissue retractor is positioned between an Anterior Longitudinal Ligament (ALL) and anterior vasculature; and advancing a cutting blade along the protective head of the tissue retractor and severing the Anterior Longitudinal Ligament (ALL) with the cutting blade.
 2. The method of claim 1, subsequent to advancing the cutting blade along the along the protective head of the tissue retractor and severing the Anterior Longitudinal Ligament (ALL) with the cutting blade, further comprising advancing an intervertebral implant through the operative corridor along the lateral, trans-psoas path.
 3. The method of claim 2, further comprising depositing the intervertebral implant into the intervertebral disc space.
 4. The method of claim 3, further comprising anchoring the intervertebral implant to at least one of a first vertebra and a second vertebra.
 5. The method of claim 4, wherein the intervertebral implant proximal end includes an extension tab that abuts a lateral aspect of at least one of the first vertebra and second vertebra, the extension tab including at least one aperture that receives a fixation anchor therethrough, wherein the step of anchoring the implant comprises advancing an anchor through the at least one aperture into one of the first and second vertebra.
 6. The method of claim 5, wherein the extension tab includes an upper portion including a first aperture that abuts a lateral aspect of first vertebra and a lower portion including a second aperture that abuts a lateral aspect of the second vertebra, and wherein the step of anchoring the implant comprises advancing an anchor through the first aperture into the first vertebra and advancing a second anchor through the second aperture into second vertebra.
 7. The method of claim 6, wherein the intervertebral implant is a fusion implant and includes at least one fusion aperture opening in the upper surface and lower surface to permit bone growth between the first vertebra and second vertebra.
 8. The method of claim 1, wherein the protective head of the tissue retractor includes an inner surface that faces the ALL and an outer surface that that faces the anterior vasculature when the tissue retractor is in the protective position.
 9. The method of claim 1, wherein the protective head of the tissue retractor includes a distal end that is curved around the anterior aspect of the target site and protects the anterior aspect of the target site from the cutting blade.
 10. The method of claim 1, wherein the tissue retractor includes a shaft extending from the protective head of the tissue retractor and a handle extending from the shaft.
 11. The method of claim 10, wherein the shaft is hollow and the cutting blade travels through the hollow shaft.
 12. The method of claim 1, wherein the cutting blade is situated on a distal end of an extender.
 13. The method of claim 1, wherein the cutting blade is situated on a distal end of an extender between a first and a second finger.
 14. The method of claim 13, wherein the second finger curves along an anterior side of the ALL as the distal head is advanced, thereby shielding the tissue lying anterior to the second finger from the cutting blade.
 15. The method of claim 13, wherein the cutting blade is disposable and the extender is reusable.
 16. The method of claim 1, wherein creating the operative corridor along the lateral, trans-psoas path the access system comprises using at least one dilator and a retractor that slides over the at least one dilator in a first configuration, and there after adjusts to a second configuration to enlarge the operative corridor.
 17. A method for correcting sagittal imbalance of a spine, the method comprising: creating an operative corridor along a lateral, trans-psoas path to a target site of the spine; preparing an intervertebral space at the target site of the spine, the intervertebral space being at least partially defined by an anterior aspect, a posterior aspect, and opposing first and second lateral aspects; advancing a tissue retractor through the operative corridor along the lateral, trans-psoas path to a protective position in which a protective head of the tissue retractor is positioned between an Anterior Longitudinal Ligament (ALL) and anterior vasculature, wherein the protective head of the tissue retractor curves to complement anterior curvature of the target site of the spine; and advancing a cutting blade along the protective head of the tissue retractor and severing the Anterior Longitudinal Ligament (ALL) with the cutting blade, wherein the cutting blade does not extend beyond the protective head of the tissue retractor.
 18. The method of claim 17, wherein creating the operative corridor along the lateral, trans-psoas path the access system comprises using at least one dilator and a retractor that slides over the at least one dilator in a first configuration, and there after adjusts to a second configuration to enlarge the operative corridor.
 19. The method of claim 17, subsequent to advancing the cutting blade along the along the protective head of the tissue retractor and severing the Anterior Longitudinal Ligament (ALL) with the cutting blade, further comprising advancing an intervertebral implant through the operative corridor along the lateral, trans-psoas path.
 20. The method of claim 17, further comprising depositing the intervertebral implant into the intervertebral disc space. 